My experience with MJ and driving is that it does not effect your motor control skills like alcohol does, but it most certainly affects your ability to pay attention. It’s real easy to zone out while stoned, and that could easily result in an accident.
I look at it like this. I would not go to the shooting range and handle a weapon drunk or stoned or otherwise impaired, so it does not make any sense to get behind the wheel of a car, which could also be considered a weapon, drunk or stoned.
You might well be able to handle it 99/ 100 times just fine, but that one time you screw up and kill someone because you got behind the wheel messed up you can’t take back.
I’ll give you a classics
Normally, when making a left turn on to a busy road. You look left, then right, then left again and proceed with caution.
When stoned, you’re liable to look left, then right, then left again, then right, and then start pulling out thinking, “I hope I didn’t miss anything”.
When you’re drunk, you think your exceptional peripheral vision and lightening reflexes has that all covered.